1. Field of the Invention
The present disclosure relates to a method of generating nanoparticles using a radio frequency plasma torch and a nebulizer.
2. Description of the Related Art
Over the last few decades, many different techniques have been utilized for the production of fine particles of a variety of materials. Mechanical attrition, wet chemical synthesis routes and aerosol/vapor based syntheses are the most commonly cited processes for this purpose. There are several advantages in using aerosol/vapor based processes like flame spray pyrolysis [1], combustion synthesis [2] and thermal plasma synthesis [3-8]. For example, they allow easy control of morphology and a relatively large production capacity. In addition, unique material phases can sometimes be obtained due to the non-equilibrium nature of these processes. Among aerosol/vapor phase processes, radio frequency (RF) plasma techniques (alternatively, inductively coupled or induction plasma) are especially valuable. RF plasma techniques offer various advantages such as large plasma volume, low gas velocity, and the capability for axial injection of the feedstock. Because the RF plasma techniques are inherently electrodeless, they also serve to minimize contamination during particle synthesis. Examples of RF plasma techniques include spray-inductively couple plasma synthesis (SICP) [9, 10], suspension plasma spraying (SPS, see U.S. Pat. Nos. 5,609,921 and 6,994,837, and U.S. Patent Publication No. 2007/0029291), and precursor plasma spraying (PPS) [7]. These techniques are useful in the synthesis of various metal, ceramic and semiconductor powders.
In the SICP process, particles of various materials are produced using a RF plasma by feeding pre-atomized solution/suspension droplets into the plasma region via a thin cooled probe inserted directly into the plasma [10, 11]. Since the probe diameter is severely constricted (less than about 1/10 of the diameter of the central core region of the plasma torch), the probability for the pre-atomized droplets to coalesce and form larger droplets is high. This can lead to the formation of large particles along with smaller ones, which not only broadens the particle size distribution, but also increases the total number of the particles having larger diameters.
The SPS (and the similar PPS) process for producing films or particles takes advantage of a high-pressure water-cooled two-fluid type probe atomizer that is inserted directly into the plasma torch [12-14]. This atomizer arrangement is not capable of producing small enough droplets. The size range of the droplets produced by the atomizer in the SPS process as stated in the specifications from manufacturer is 20-120 μm or larger depending on operating conditions. Furthermore, the arrangement of having the probe inserted directly into the plasma makes it impossible to attach a scavenging mechanism for removing large droplets. As a result, particles with a wide size range ranging from a few tens of nanometers to larger than 10 μm are produced. This is easily appreciated by perusing the size distribution data provided in references [8] and [14]. Although the current state-of-the-art RF plasma-based techniques can produce particles in the sub-100 nm range of diameters, they are inherently incapable of producing a plurality of nanoparticles with a majority of the volume concentrated in sub-100 nm particles. From the point of view of industrial production, it is important to have the ability to produce a large volume fraction carried by sub-100 nm particles. A large fraction of the mass of particles produced by current industrial processes now would have diameters larger than 100 nm, which would have to be rejected in nanoparticle production.
Various embodiments described herein address the need for a technique to successfully control the particle size distribution below the 100 nm threshold by using RF plasma processing of atomized solvent/suspension precursors. Furthermore, this technique can also be applied to other processes like flame spray synthesis, combustion synthesis or any system where an aerosolized form of the precursor needs to be injected into a reaction zone for particle synthesis.